The principle of the method is a that of a peritectic-type growth method based on a two-phase medium, which method is usually performed in a crucible. Before solidification proper, that method requires high treatment temperatures, typically at least 50.degree. C. higher than the peritectic solidification temperature T.sub.p of the oxide in question (e.g. 1,000.degree. C. for YBaCuO). At such temperatures the superconductive oxide is decomposed into a solid (e.g. the solid 211 phase Y.sub.2 BaCuO.sub.5 for YBa.sub.2 Cu.sub.3 O.sub.7-x) and a liquid which might react with the walls of the crucible. This results in drift in the composition of the phase diagram.
Solidification is very slow, in particular, because the reaction involves two solid bodies and one liquid body; e.g. for YBaCuO: ##STR1##
(where x=0.5).
Ideally, ingot growth should take place in the vicinity of the peritectic temperature T.sub.p with a small amount of supercooling; a large amount of supercooling would lead to solid particles being included in the growth front. Moreover, growth should take place uniformly, in a non-convective state, because local variations in supercooling would give rise to high instantaneous growth speeds, with the risk of solid particles being trapped in the growth front.
The ideal method of growing an ingot should therefore be a slow-growth diffusion method, but that would involve the liquid and the crucible being in contact with each other for very extended contact times at very high temperatures, with the damaging consequences that are described above: partial dissolution of the crucible, modification in the phase diagram, inclusion of parasitic phases, precipitates at grain boundaries, etc., the result being that the texturing of the final ingot is partially destroyed, and that large-size precipitates are present therein. In addition, cracks are formed due to the difference in the coefficients of expansion of the crucible and of the ingot, with the ingot sticking to the crucible.
All these adverse effects ultimately lead to a reduction in the critical current density J.sub.c and prevent large-size ingots, e.g. of about 50 cm.sup.3, from being made.